What other movie was a man returned as a dog to solve his murder and he had a helper that had been a horse she liked to eat oats and ran real fast it is an older movie maybe about 1950?

The movie you're thinking of is likely "The Shaggy Dog," released in 1959. In this Disney film, a teenage boy is transformed into a sheepdog and must navigate various challenges, including solving a mystery involving his own transformation. While it doesn't feature a horse as a helper, it embodies the whimsical tone and themes of animal transformations. If you meant a different film, please provide more details!